Detecting vehicle wheels is of interest for numerous applications. Thus it is possible to infer with certainty from the recognition of wheels that a given traffic area is being driven on in order, for example, to monitor borders or to initiate certain actions such as triggering an alarm, switching on lighting, opening a barrier, taking a picture for monitoring purposes, etc. Modern traffic fee systems also frequently base the calculation of fees on the number of axles of vehicles, so that the detection of wheels (wheel axles) can also be an important basis for road toll or parking fee systems.
Detecting wheels of a moving vehicle based on the horizontal component of their tangential velocity, which differs from that of the rest of the vehicle and brings about a corresponding Doppler frequency shift of a radar measuring beam, is known, for example, from DE 10 2008 037 233 A1. For this purpose, a radar speed measuring unit is used that irradiates the lower area of passing vehicles with a radar beam lobe and, from the returning frequency mixture, determines a single speed measurement signal that has signal maxima at the locations of the wheels. Automatic detection of such maxima in a signal curve requires a signal-analytical search for extreme values and is correspondingly elaborate. In addition, gaps between a traction vehicle and its trailer can falsely indicate signal maxima and intermediate “false” maxima, which lead to an erroneous wheel detection.